I went to a health care facility last Saturday to do a ground impedance test with an Amprobe INSP-3, so we randomly checked 30 or so outlets. With this meter the highest impedance was about 9.8, some of the low impedance were around 1.3. So I started trying to improve this by removing the grounding electrode from the building steel, wire brushing the building steel and the conductors, also removed the conductor from the cold water pipe and the ground rod and repeated the process. Checked again with the meter and the highest was still 9.8. This plug was out of an IG panel, so I removed the cover and connected a piece of #12 to the isolated ground bar, then ran the other end approximately 175 feet to that outlet. I removed the outlet from the wall, removed the green/yellow wire from the plug and installed the wire that I had laid out on the floor to the outlet and nothing changed the reading was still the same. I removed the wire from the isolated ground bar and installed a receptacle at the panel, I connected the green wire to the receptacle at the panel, and brought the other end to where the cold water pipe was at, disconnected the wire going to the building steel and connected my #12 to the cold water pipe. The results did not change. I reconnected the wire to the cold water pipe along with my #12 and that did not change the outcome either. I removed my #12 from the cold water pipe and took it to the electrical room. I installed it under the acorn clamp along with the conductor that ran over to the panel. Nothing changed. I then disconnected the wire going to the ground rod and connected the #12 to the ground rod only. It read .73 They want the impedance to be 0.2 ohms for systems containing isolated ground receptacles and 0.1 for all others. Any help or information would be greatly appreciated. The service was 800 amps at 480 volts.
Ground impedance test
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K8MHZ
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From Table 9, insulated #12 is 2 ohms per 1000 feet, or .2 per 100 feet. 175 feet would be .35 ohms. To get down to .1, the distance is shortened to 50 feet.
That does not take into consideration other factors, such as termination resistance.
That does not take into consideration other factors, such as termination resistance.
Sparky3141
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I had never used one of those testers before a customer complained of high ground impedance in the receptacles feeding their server equipment. They were using the Ideal brand and is all it would tell them is that it was over about 5 ohms or something like that and that was way too high. My understanding was that it was reading the impedence back to the source along the effective ground fault current path.
Understanding this helped me know where to start looking. I opened the panel and found a supply side bonding jumper but no GEC or system bonding jumper. So I proceeded to the source, the transformer, and did not find a system bonding jumper there either. So all I had to do was correctly bond the system and it brought the Z down to under 1.
My point is that I think you are barking up the wrong tree by even looking at the structural steel and waterpipe bonds as they have absolutley nothing to do with the effective ground fault current path of your receptacles. Just make sure you know how your circuit is routed and clean up all your splices and make sure your step down transformers are bonded correctly. You don't need to go beyond the seperately derived system to the 480 volt side at all.
Understanding this helped me know where to start looking. I opened the panel and found a supply side bonding jumper but no GEC or system bonding jumper. So I proceeded to the source, the transformer, and did not find a system bonding jumper there either. So all I had to do was correctly bond the system and it brought the Z down to under 1.
My point is that I think you are barking up the wrong tree by even looking at the structural steel and waterpipe bonds as they have absolutley nothing to do with the effective ground fault current path of your receptacles. Just make sure you know how your circuit is routed and clean up all your splices and make sure your step down transformers are bonded correctly. You don't need to go beyond the seperately derived system to the 480 volt side at all.
Sparky3141
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Please read 250.96 (B) which will also take you to 250.146 (D) only a paragraph each but states very clearly that the IG is to terminate at an equipment grounding conductor terminal (bar) of the applicable derived system or service.
It is not supposed to be isolated from the panelboard that also serves as the first disconnecting means from the transformer as that is the separately derived system that is part of your effective ground fault current path. The seperately derived system is supposed to be bonded to the structural steel and it is also supposed to be bonded to the metal water pipe system and it is also supposed to be earth grounded.
But don't let any of that confuse you; the IG is also supposed to be bonded to all of these as well as to the supply side bonding jumper and/or the main bonding jumper (if it's a service) or system bonding jumper (for a separately derived system) in order for the ground fault path to effectively return to its source which is what's needed to clear a fault.
If you you can get a hold of an NEC Handbook there's a pretty good explanation of
250.96 (B) and 250.146 (D)
It sounds to me that you are mistakenly thinking that your "ground impedance" test is somehow measuring the earth ground when what it is measuring is the EGC back to the system it came from via all of the required bonding to the system neutral.
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